This invention relates to a waveguide type optical device, such as a waveguide-type optical modulator and a waveguide-type optical switch, used in various optical systems including high-speed optical communication, optical switching network, optical information processing, and optical image processing.
A waveguide-type optical modulator and a waveguide-type optical switch are important components to compose various optical systems including high-speed optical communication, optical switching network, optical information processing, and optical image processing. Especially a modulator using a LiNbO3 substrate is a promising device since it has a smaller wavelength chirping in modulation than that of a semiconductor-system modulator, e.g., a modulator using a GaAs-system substrate.
Important parameters to determine the performance of LiNbO3 optical modulator are drive power (or drive voltage), modulation bandwidth and insertion loss. Of these parameters, the modulation bandwidth and drive voltage are in trade-off relationship. Therefore, it is difficult to widen the modulation bandwidth as well as lowering the drive voltage. So, searches about waveguide-type optical modulator focus on the optimization of the trade-off relationship.
The bandwidth of waveguide-type optical modulator is mainly dependent on the kind, material and placement of electrode, and the permittivity of substrate. So, in order to widen the bandwidth of waveguide-type optical modulator, a traveling wave electrode is in wide use, and is formed as an extension of transmission line. Here, the characteristic impedance of electrode has to be equal to that of microwave power source and load. In this case, the modulation speed is restricted by the difference between the traveling times (or phase speeds or effective refractive indexes) of light wave and microwave. Meanwhile, as the traveling wave electrode structure used widely, there are two kinds of structures, i.e., an asymmetric strip line (hereinafter referred to as xe2x80x98ASLxe2x80x99) type or asymmetric coplanar strip (hereinafter referred to as xe2x80x98ACPSxe2x80x99) type electrode structure, and a coplanar waveguide (hereinafter referred to as xe2x80x98CPWxe2x80x99) type electrode structure.
The bandwidth of modulator is restricted by microwave attenuation xcex1, the speed discordance or effective refractive-index difference between light wave and microwave. To suppress the speed discordance, characteristic impedance and microwave attenuation, it is necessary to optimize the buffer-layer parameter and electrode parameter, particularly the width of signal electrode and the interval between signal electrode and earth electrode. However, even if the speed discordance could be suppressed, the bandwidth of modulator is restricted by microwave attenuation. So, to suppress the microwave attenuation is most important for realizing the wider bandwidth of modulation. Moreover, by reducing the microwave attenuation, the drive voltage in trade-off relationship with the bandwidth can be also controlled at the same time.
The microwave attenuation is caused by phenomena below.
(a) a loss in strip-line conductor that is a function of the form or structure of electrode (width of signal electrode, interval between signal electrode and earth electrode etc.), the resistivity of electrode material, buffer-layer parameter etc.
(b) a dielectric loss that is a function of the permittivity of LiNbO3 substrate and tan xcex4 (loss tangent)
(c) a loss due to higher-order mode propagation
(d) a loss due to the impedance discordance between power-supply side characteristic impedance and load side characteristic impedance (normally, both characteristic impedances are matched into 50 xcexa9)
(e) a loss in strip-line curved portion and tapered portion
(f) a loss due to a mounting package and external package including a loss in a connector, a feeder part of signal electrode, connection method or material thereof.
About the above phenomena (a), (b), (c) and (d), the optimization of electrode parameter and buffer-layer parameter has been considered to some extent. The inventor of this application also discloses an optical modulator that using a thick CPW electrode structure, a bandwidth as wide as 20 GHz and a drive voltage as low as 5V are obtained, in xe2x80x9cA Wide Band Ti:LiNbO3 Optical Modulator with A Conventional Coplanar Waveguide Type Electrodexe2x80x9d, IEEE Photonics Technology Letters, Vol. 4, No. 9 (1992) (first prior art).
Adding to this, various optical modulators using ASL/ACPS type electrode structure or CPW electrode structure are suggested. The typical examples are disclosed in xe2x80x9cTraveling-Wave Electro-Optic Modulator with Maximum Bandwidth-Length Productxe2x80x9d, Applied Physics Letters, Vol. 45, No. 11, pp. 1168-1170 (1984) (second prior art), xe2x80x9c20-GHz 3 dB-Bandwidth Ti:LiNbO3 Mach-Zehnder Modulatorxe2x80x9d, International Conference, ECOC""90 pp. 999-1002 (1990) (third prior art), and xe2x80x9cHighly Efficient 40-GHz Bandwidth Ti:LiNbO3 Optical Modulator Employing Ridge Structurexe2x80x9d, IEEE Photonics Technology Letters, Vol. 5, No. 1, pp. 52-54 (1993) (fourth prior art).
In general, an electric band (S21 characteristic) of modulator is represented as below.
xcex1=xcex10(f)xc2xdL 
where xcex1 is a microwave loss (or microwave attenuation) of all electrodes [dB], xcex10 is a microwave attenuation constant [dB/{cm(GHz)xc2xd}], f is a frequency [GHz], and L is an electrode length [cm].
The above electric band (frequency for S21-characteristic of 6 dB) is restricted by the microwave attenuation constant xcex10 of electrode, and further influenced by the optical characteristic. Thus, the reduction of microwave attenuation constant xcex10 of electrode is restricted by the entire bandwidth of device. Meanwhile, the values of microwave attenuation constant xcex10 of electrode in the above prior arts are 0.45 (first prior art), 3.75 (second prior art), 0.5 (third prior art) and 0.75 (fourth prior art).
However, in order to construct a further high-speed communication system for, e.g., 40 Gb/s, it is necessary to realize an optical modulator with a wide modulator band of 30 GHz or wider and a low drive voltage of 3.5 V or lower. Therefore, the microwave loss has to be further reduced.
Referring to FIGS. 1A and 1B, an example of waveguide type optical device, which is disclosed in the first prior art, is explained below. FIG. 1A is a plan view showing the conventional waveguide type optical device, and FIG. 1B is a cross sectional view cut along the line Gxe2x80x94G in FIG. 1A.
In the conventional waveguide type optical device in FIGS. 1A and 1B, a titanium metal film strip is formed on a crystal substrate 101 with electro-optic effect, and, by internally-diffusing titanium into crystal of the crystal substrate 101, an incidence-side Y-branch waveguide 102, an emission-side Y-branch waveguide 103 and a phase shifter waveguide 104 are formed on the crystal substrate 101. Namely, on the crystal substrate 101, the two Y-branch waveguides to function as the incidence-side Y-branch waveguide 102 and emission-side Y-branch waveguide 103, and the phase shifter waveguide (Mach-Zehnder interferometer type) 104 with two arms are provided.
Also, on the crystal substrate 101, a buffer layer 105 composed of a dielectric material is formed. On the buffer layer 105, a CPW type electrode structure composed of one signal electrode 106 (107) and two earth electrodes 108 and 109 is formed. On the incidence and emission sides of the waveguide, optical fiber mounts 110a and 110b, respectively, are provided. Further, to the optical fiber mounts 110a and 110b, optical fibers 111a and 111b, respectively, are connected.
In operation, optical field (ray of light) propagated through the optical fiber 111a passes through the optical fiber mount 110a, being input to the incidence-side Y-branch waveguide 102, propagating through the phase shifter waveguide 104 and emission-side Y-branch waveguide 103, then passing through the optical fiber mount 110b, being output to the optical fiber 111b. 
In this process, incident light is divided into two equal parts (light waves) by the incidence-side Y-branch waveguide 102, and propagated through the two arms of the phase shifter waveguide 104. When the phase shifting is not applied between the two arms of the phase shifter waveguide 104, i.e., when no external voltage is applied between the two arms, two light waves are in phase connected by the emission-side Y-branch waveguide 103, output to the optical fiber 111b without reducing the optical output intensity. On the other hand, when the phase shift xcfx80 is given between the two arms by applying each external voltage, two light waves are subject to the compensating interference in the emission-side Y-branch waveguide 103, thereby the optical output intensity (intensity of light output from the emission side) becomes a minimum value or zero.
Thus, by applying the external voltage, light wave passing through the waveguide type optical device can be turned ON or OFF. So, the switching or modulation control of light wave passing through the waveguide type optical device can be performed. Also, by applying so called high-frequency microwave between the two arms of the phase shifter waveguide 104 by using the external voltage, the concerned waveguide type optical device can operate as a high-band optical modulator. In this case, for example, with an electrode length of 3 cm, a microwave attenuation constant xcex10 of 0.5 dB/{cm(GHz)xc2xd} and a drive voltage of 4 V, the electric band (frequency for S21-characteristic of 6 dB) is 16 GHz.
However, as described earlier, in order to construct a further high-speed communication system for, e.g., 40 Gb/s, it is necessary to realize an optical modulator with a modulator band as wide as 30 GHz or wider and a drive voltage as low as 3.5 V or lower. For that purpose, the microwave attenuation constant xcex10 needs to be 0.37 dB/{cm(GHz)xc2xd}. Namely, the microwave loss must be reduced about 26%, comparing the above example for a microwave attenuation constant xcex10 of 0.5 dB/{cm(GHz)xc2xd}. Therefore, it is necessary to further reduce the microwave loss (particularly loss in strip line conductor of traveling wave electrode structure) as well as further reducing the drive voltage.
In general, in case of a low electrode resistivity, microwave propagates through the electrode without reducing much. So, by further reducing the electrode resistivity, the microwave loss in the entire electrode can be reduced, thereby allowing the waveguide type optical device (modulator) to provide a further wide bandwidth and high-speed operation.
For example, the electrode resistivity R is given by:
R=xcfx81 L/A 
where xcfx81 is a specific resistivity of electrode material, L is a length of electrode and A is an area (=electrode widthxc3x97electrode thickness) of electrode. Thus, the larger the electrode area A is or the smaller the specific resistivity xcfx81 is, the smaller the electrode resistivity R is.
As explained earlier, the bandwidth of modulator is restricted by microwave attenuation, the speed discordance or effective refractive-index difference between light wave and microwave. To suppress the speed discordance and the characteristic impedance, it is necessary to design to optimize the buffer-layer parameter and electrode parameter. So, at the stage of designing, the parameters are determined. Namely, at the stage of designing, the size of electrode area A, as well as the value of electrode resistivity R, must be determined. Meanwhile, the electrode length L is determined by the trade-off relationship between drive voltage and bandwidth.
Since the electrode parameters are thus determined already, it is difficult to further reduce the loss in strip line conductor of traveling wave electrode structure. So, the remaining means to further reduce the loss in traveling wave electrode structure is to change the resistivity of electrode material. The electrode material used thus far is limited to gold, copper or the like mainly due to its low specific resistivity xcfx81. The specific resistivity xcfx81 of gold is 2.05xc3x9710xe2x88x926 xcexa9cm at a temperature of 0xc2x0 C., 2.15 to 2.2xc3x9710xe2x88x926 xcexa9cm at 20xc2x0 C., and 2.88xc3x9710xe2x88x926 xcexa9cm at 100xc2x0 C. For example, with an electrode length L of 4 cm, an electrode width w of 7 xcexcm, an electrode thickness of 25 xcexcm and a specific resistivity xcfx81 (gold, 20xc2x0 C.) of 2.15xc3x9710xe2x88x926 xcexa9cm, the electrode resistivity is 4.9 xcexa9.
Accordingly, it is an object of the invention to provide a waveguide type optical device that realizes a wider bandwidth and a higher-speed operation.
According to the invention, a waveguide type optical device with a traveling wave electrode structure, comprises:
a crystal substrate with electro-optic effect;
an optical waveguide that is formed on the crystal substrate and waveguides light wave;
a buffer layer that is formed on at least the optical waveguide and is of a dielectric material; and
a signal electrode and an earth electrode that are formed on the buffer layer and control the optical output intensity of light wave waveguided through the optical waveguide;
wherein the traveling wave electrode structure is composed of the one signal electrode and the two earth electrodes that are disposed sandwiching the signal electrode, at least the signal electrode of the traveling wave electrode structure is of a material that has a specific resistivity of 2.1xc3x9710xe2x88x926 xcexa9cm or lower at an environmental temperature that the waveguide type optical device is used.
According to another aspect of the invention, a waveguide type optical device with a traveling wave electrode structure, comprises:
a crystal substrate with electro-optic effect;
an optical waveguide that is formed on the crystal substrate and waveguides light wave;
a buffer layer that is formed on at least the optical waveguide and is of a dielectric material; and
a signal electrode and an earth electrode that are formed on the buffer layer and control the optical output intensity of light wave waveguided through the optical waveguide;
wherein the traveling wave electrode structure is composed of the one signal electrode and the one earth electrode that is disposed pairing with the signal electrode, at least the signal electrode of the traveling wave electrode structure is of a material that has a specific resistivity of 2.1xc3x9710xe2x88x926 xcexa9cm or lower at an environmental temperature that the waveguide type optical device is used.